pH-dependent electrostatic interactions between enzymes and nanoparticles in Pickering emulsions - Influence on activity and droplet size.

Pickering emulsions (PE) are liquid-liquid systems that are stabilized by solid (nano)particles at the fluid interface. They offer higher stability, easier separation and lower toxicity compared to classical emulsions stabilized by surfactants. Common applications range from food science to drug delivery. In the last decade they have become of more interest in the field of multiphasic biocatalysis. First, this study aims to present the influence of pH, salt strength and particle charge on enzyme activity. The different behavior of two lipases (CaLA and CRL) is shown. While the activity optimum of CaLA changed from pH 6.5 to pH 5 by applying particles with negative instead of positive surface charge, the CRL activity optimum stayed at pH 5-5.5. This enables particle charge as an additional parameter to optimize biocatalytic reactions in PEs. Second, the resulting drop sizes were measured to elucidate further interactions between the enzymes and particles in PEs. Drop sizes in PEs prepared with CaLA were not influenced by pH, but increased for positively and decreased for negatively charged particles upon the addition of CaLA. Electrostatic attraction between particles and CRL increased the droplet diameter from 10 μm up to 30 μm and therefore destabilized the PE for both particle types.